Wireless communication systems have evolved drastically from the first voice-only cellular telephone systems to high-speed digital data networks capable of delivering voice, Internet, and even streaming video content to miniature handheld units that users carry with them and use as they travel.
In a typical cellular radio communication system (wireless telecommunication network), an area is divided geographically into a number of transmission areas, or cell sites, each of which is defined by one or more radio frequency (RF) radiation patterns from respective base transceiver station (BTS) antenna. The BTS of a cell may also be called an Access Point (AP) which implements functions of a BTS. A cell may refer to a coverage area serviced by an AP. A cell may further include one or more sectors.
When a non-stationary mobile user equipment also called an Access Terminal (AT), is positioned in a cell site, it communicates wirelessly with Access Points (APs). The APs communicate with a network service node of an Access Network (AN) of the telecommunication network.
An Active AT may have one or more sectors in its active set. An AP may have multiple sectors, making it possible for the active set to contain more than one sector for the same AP, or “cell”. The AT receives forward link data from one sector at a time. The AT sends a signal to the AN via AP that indicates the best serving sector among the sectors in the active set. This signal is called the Data Rate Control (DRC) signal. The AT is said to “point” its DRC to the sector from which it may receive data next. The DRC signal also indicates to the AN the rate at which data should be sent to the AT. The AT generally varies the DRC signal based on measurements taken at the AT of one or more forward link signal parameters. Where the AP has multiple sectors, the AT may also identify in the DRC signal a specific sector from which the AP may transmit data to the AT. If the source and target sectors belong to the same AP, then the AT may switch between them without incurring any delay.
A wireless communication system permits an AT to move from the coverage area of one AP to that of another AP without losing the AT's connection with the AN. At any given time, the AT in a wireless network will typically be linked to a given sector and will be able to communicate with the telecommunication network via the AP serving that sector. The AN sends data from the AP at the rate requested by the AT. If the AT intends to move into a new sector of another AP, the AT attempts to switch from the first AP to the second AP serving the new sector. The process of rerouting data for an AT from a first AP to a second AP is called handoff. The AT sends a DRC signal requesting to switch the serving sector from a source sector in a first AP to a destination (target) sector in a second AP.
When the AT moves from the coverage area of one AP to another, the handoff process occurs because of the AT's transition from an initial sector of a cell site to a target sector of a different cell site. The AT may communicate with the new sector of second AP once the handoff process from the initial sector in the original AP to the target sector in the second AP is complete.
Wireless standards provide specifications that promote compatibility between various manufacturers of ATs and AN equipment. One such wireless standard for wireless high rate packet data (HRPD) systems, is as specified in “cdma2000 High Rate Packet Data Air Interface Specification,” 3GPP2 C.S0024-0 Version 4.0, published in 2002, referred to herein as a “1xEV-DO (1x Evolution-Data Only) Release 0 network”.
A soft handoff takes place when the AT switches from one sector to another sector in the same AP, or when the AT moves from one AP to another AP wherein the APs have a common frequency assignment. Soft handoff (handoff between two APs) switching causes a short period of outage. When the source and target sectors belong to different APs with different frequency assignment, during the AT handoff from one AP to another AP, the AT stops communicating with the first AP and starts communicating with the second AP after a certain time period (delay) sufficient for the AN to reroute forward link data to the second AP associated with the target sector in a hard handoff. The outage ranges from 100 milliseconds to as high as 200 milliseconds. This delay is because the AN takes time to re-route data packets targeted for the source BTS to the target BTS. Even though the AT has pointed its DRC to a sector of the second AP, the network will not be able to transport data to or from the AT through the second AP until after this delay or switch time.
For some applications, the handoff process or the rerouting process may result in a brief interruption in the data flow or data communication with the AT. Thus, handoff of an AT from a source AP to a target AP may interrupt signal transmission and reception between the AT and the AP, and may cause unacceptably long interruptions to services such as Voice over IP (VoIP), video telephony, network gaming, or other applications requiring high Quality of Service (QoS). Most de-jitter buffers on mobile VoIP handsets are able to handle 20-30 milliseconds delay and make it undetectable to the voice user on a call, but 100-200 milliseconds delay is beyond the capability of most de-jitter buffers and voice quality suffers.
The TIA-856-A standard, entitled “cdma2000 High Rate Packet Data Air Interface Specification,” 3GPP2 C.S0024-A, published by the Telecommunications Industry Association in 2004, referred to herein as a “1xEV-DO Revision A network”, describe methods of minimizing the duration of the interruption of data communication during handoff of an AT. The delay caused due to rerouting process of the AN is improved in revision A of the 1xEV-DO standard, with the introduction of a Data Source Control (DSC) switching. The forward sector switching in EV-DO is enhanced with the introduction of DSC channel. The DSC channel allows the AT to send an early indication of an impending handoff to the base station. The DSC signal is sent from the AT to the AN, and indicates the cell to which the AT will point its DRC after a specified delay time called “DSCLength”. When an AP receives a DSC signal from an AT indicating that the AT will handoff to a new AP, the AP forwards the information to one or more entities in the AN through a Backhaul Connection. The AP then participates in the synchronous switching of routing of data to the AT. Thus, the DSC signal provides advance notification to the AN of the AT's intention to point its DRC at a sector belonging to the target cell, thereby minimizing the interruption in data flow to the AT caused by the handoff.
During BTS switching, the addition of DSC functionality enables the Access Network (AN) to send data packets to a target cell at almost the same time Access Terminal (AT) switches to target BTS, hence minimizing cell switching outage.
The DSCLength normally chosen is such that it provides sufficient time for the AN to reroute forward link data from the source AP to the target AP identified in the DSC signal. DSCLength is measured in slots. Alternatively, DSCLength may be measured in some other time differential, or may be identified by some other techniques such as by indicating an index into a lookup table of possible time periods.
Generally, AT uses a default value for DSCLength. Alternatively, the AN may also provide the DSCLength parameter to the AT prior to the DSC signal, to be used in the event of a switch to another AP. The AN may optionally adjust the DSCLength parameter used for handoff to be long enough to seamlessly switch to a sector of another AP. The AN sets the DSCLength to a relatively shorter value, if the AT needs to handoff to a target sector that is in the same cell as the source sector. In the event of AT performing a handoff to a target sector in a different cell than the source sector, the AN sets the DSCLength to a larger value to accommodate the longer time needed to reroute data to a different cell. After the time period specified in DSCLength passes, the AT directs its DRC signal to the target sector and it may then receive data from said target sector.
The “service outage” time during cell switching is dependent on the DSCLength which in turn is dependent on the Backhaul Delay (between the AN and BTS). DSCLength specifies the minimum time required prior to the AT switching cells. This time should be greater than the Backhaul Delay. However, if this time is much larger than the Backhaul Delay, the handoff outage is increased. Thus, although the interruption in the data flow caused by handoff is tried to be minimized by the AN adjusting the DSCLength, a challenge lies in reducing cell switching (handoff) outages which pose a problem for delay-sensitive applications, such as VoIP data. A short DSCLength is desirable to reduce the delay and possible service degradation due to bad channel conditions.
Accordingly, it would be desirable to reduce handoff outage between an AT and an AP, as much as can be possible.